The present invention relates to the manufacture of 2,2,2-trifluoroethanol by hydrogenolysis of 2,2,2-trifluoroacetaldehyde (fluoral) hydrates or hemiacetals.
An important industrial source of fluorinated primary alcohols such as 2,2,2-trifluoroethanol, which has uses in a wide variety of applications such as energy recovery (absorption heat pumps), pharmaceutical products (anaesthetics) and solvents, is based on the reduction of the corresponding acid (in this case trifluoroacetic acid) or of a derivative (ester, acid chloride, anhydride, amide) with hydrogen in the presence of a catalyst which is chosen in most cases from the precious metal group (rhodium, ruthenium, platinum, palladium). Among the chief techniques employed, there may be mentioned the hydrogenation of trifluoroacetic anhydride (U.S. Pat. No. 4,255,594), the hydrogenation of trifluoroacetic acid (Patents: U.S. Pat. No. 4,273,947, FR 2,544,712, and FR 2,545,481), the hydrogenation of esters of trifluoroacetic acid (Patent EP 36,939), the hydrogenation of trifluoroacetamide [M. Gilman, J.A.C.S., 70, 1281-2 (1948)], and the hydrogenolysis of trifluoroacetyl chloride (U.S. Pat. No. 3,970,710). Apart from the disadvantage of poor catalyst behavior with time, these processes have the economic disadvantage of relying on an oxidation of the starting materials (chlorinated in most cases), to provide access to the acid or to one of its derivatives, followed by a reduction of this acid to alcohol; this additional stage represents a very serious burden on the profitability of these processes.
Another group of processes consists in hydrogenating fluoral or one of its derivatives. The yield produced by liquid phase hydrogenation (80.degree. C. at 95 bar) of fluoral hydrate over a nickel catalyst (Patent FR 1,399,290) is indifferent; the catalyst life is not mentioned and the reaction requires a large quantity of catalyst (16% by weight of pure nickel, based on the fluoral) under very severe operating conditions. Mention can also be made of U.S. Pat. No. 2,982,789, which describes the gas phase hydrogenation of fluoral hydrochloride: CF.sub.3 CH(Cl)OH originating from a first stage of a hydrogenolysis (Rosenmund reaction) of trifluoroacetyl chloride over a palladium catalyst. The fluoral hydrogenation catalyst, which consists of copper chromite deposited onto calcium fluoride, and whose behavior with time is not mentioned, operates at about 250.degree. C. and enables the fluoral intermediate to be converted only incompletely (approximately 60-65%); in addition, recycling the unconverted fluoral hyrochloride is a highly hazardous operation because of its thermal instability, the decomposition: ##STR2## being promoted at 30.degree. C. and above by a temperature rise or a pressure reduction. Lastly, there may be mentioned the gas phase hydrogenation of fluoral (U.S. Pat. No. 3,468,964) in the presence of a palladium catalyst deposited onto alumina at a low temperature (peak temperature: 140.degree. C.); the relatively poor yield of trifluoroethanol (86%) and the need to regenerate the catalyst very frequently at 200.degree. C. in pure oxygen as well as the extreme difficulty of conveying fluoral in a pure state (polymerization) make the process unattractive.